For safety and economic reasons, aircraft must have the capability of determining accurately the amount of fuel on board. Federal regulations determine the minimum amount of fuel which an aircraft must carry because unexpected events may alter the course and destination of the aircraft. In these situations it is important to have an accurate measurement of fuel remaining to enable the craft to be flown safely to an alternate destination.
The amount of fuel to be carried by an aircraft is determined by a variety of considerations, such as payload, route, weather, etc. The heavier the fuel load, the less payload an aircraft can carry, and error in fuel gauging is often accounted for by additional fuel reserves. Therefore, accurate on-board fuel gauging equipment is commercially desirable because it allows less fuel to be carried for a given route, resulting in a more economic flight operation.
Conventional fuel gauging systems have used capacitance probes located in the tanks of aircraft. Generally, such probes consist of two concentric vertically-disposed electrodes defining a space in which fuel rises during filling and falls during consumption. The level of the fuel within the probe alters the capacitance value of the probe. The capacitance of the probe varies linearly with the fuel level within the probe. Therefore, an electrical signal applied to the probe can be used to determine a probe capacitance value at a given fuel level. This capacitance value can then be used to determine the quantity of fuel in the tank.
In one type of commercially available fuel gauging system the capacitance of each probe is measured individually. Each probe is linear, i.e., the diameters of inner and outer electrodes remain constant throughout the length of the probe. Therefore, the probes are relatively inexpensive to manufacture. The integrity of each individual probe is monitored to detect probe failure. However, with such a system there is an increased complexity of the monitoring electronics and wiring. The additional wiring adds to the weight and cost of the system, and reduces system reliability. The electronics are more complicated because each probe must be driven separately in order to achieve a system reasonably responsive to fuel level changes. The added complexity makes such a system relatively more expensive and less reliable.